Bar gripping means for use in the production of prestressed concrete



Feb. 26, 1957 D. H. LEE ,78

BAR GRIPPING MEANS FOR USE IN THE PRODUCTION OF PRESTRESSED CONCRETEFiled April 1, 1954 3 Sheets-Sheet 1 4 TTOK/YEYS D. H. LEE 2,783,024 INTHE PRODUCTION ONCRETE 3 Sheets-Sheet 3 I I Illn 2 (if (III I QMM 4 v Rl h t x Q w .FN \w///////////// B w \N Feb. 26, 1957 BAR GRIPPING MEANSFOR USE OF PRESTRESSED C Filed April 1, 1954 1 L fi United States PatentBAR GRIPPING MEANS FOR USE IN THE PRO- DUCTION OF PRESTRESSED CONCRETEDonovan Henry Lee, London, England Application April 1, 1954, Serial No.420,431

Claims priority, application Great Britain May 12, 1953 Claims. (Cl.254-29) This invention relates to the prestressing of concrete and inparticular to means for gripping the prestress bars or the like for thepurpose of tensioning them.

The problems involved in devising gripping means for anchoring andholding prestress bars under the high tensile forces employed inprestressed concrete work do not merely comprise the provision of adevice strong enough to resist the tensile forces and to grip the barsso that they do not slip, but also the provision of a gripping devicewhich can be used either for post-tensioning or pretensioning, in theformer case having ready means of transferring the thrust onto theconcrete prior to removing the jack. Unless a gripping device, of thetype with which the present invention is concerned, is also a relativelycheap item to produce, it is of little value commercially howevereffective it is in carrying out its job. In this case the usefulness isincreased by the gripping means being adaptable to suit variousdiameters of bars by the use of different Wedges.

It is therefore the main object of the present invention to provide agripping device for prestress bars in the manufacture of prestressedconcrete which are highly eflicient as to their bar gripping and tensionsustaining properties but are also economic to manufacture and use andare suitable for application in a variety of different circumstances andwith varying types of apparatus, more particularly different forms ofjack.

The gripping means now to be described is applicable to bothpretensioning and post-tensioning techniques, that is to say it is notdependent on whether the concrete has been poured and has hardenedbefore the bars, or the like, are tension'ed. This invention isprimarily applicable to bars With deformed surface but can if desired beused on plain round bars.

In accordance with known practice in the making of prestressed concretemembers the steel is tensioned, either before or after the concrete hashardened. In the former case (pretensioning), steel. thrust plates orthe like are held apart by any desired form of strutting while theconcrete is placed and sets round the bar or bars. In the latter case(post-te'nsioning) the bar when tensioned and gripped reacts directlyagainst the ends of the hardened concrete.

In accordance with this invention, a bar to be tensioned by eithermethod of prestressing is gripped by a plurality of grooved or serratedwedges retained in a steel or other metal collar with a sloping ortapering internal surface which may, for example, be square tapering orconical, the collar being threaded on its external face to engage byscrewing a further threaded collar so that the latter may be turned tooccupy any desired position along the threading of the wedge retainingcollar and being further co-axiall'y threaded radially inwardly of saidexternal face threading to provide for the direct or indirect attachmentof a co-axi'al jack shaft. This further adjustable collar may havesufiicient axial length to the extent to make contact with thebeforementioned steel thrust plate or the 'end of the hardened concretemember or alternatively a cylindrical metal distance piece, for examplea sleeve or short length of pipe, may be interposed.

The inner or smaller diameter co-axial threading on the Wedge retainingcollar may be externally facing on a portion of the collar of reduceddiameter or may be internal threading on a cylindrical recess or bore inthe collar, the said inner threading being locatable remote from theface of the concrete member, when the gripping means is in situ.

The steel bar held by the gripping device is tensioned by any suitablemeans, such as a hydraulic jack, the shaft of which is connected eitherdirectly to the gripping means by the engagement of threads on the jackshaft with the aforesaid internally facing inner threads on the grippingmeans where such is provided, or indirectly by means of an interveningthreaded sleeve having threaded engagement with the inner externallyfacing threads on the gripping member and threaded on cotter-and-slotengagement with the jack shaft.

As the bar is stretched in the manner described, the adjustable collaron the outermost threading on the gripping means is screwed forward tomaintain contact with the face of the concrete member or thrust plateand when the desired tension or stretch is obtained the jacking meansmay be disconnected, the tension in the bar being taken by the collar onthe steel thrust plate or the hardened concrete member as the case maybe, While the concrete hardens in the case of pretensioning, or whilegrout injected along the length of the duct in which the bar is placedhardens in thecase of post-tensioned concrete.

In the accompanying drawings:

Figure l is a sectional side elevation of one form of gripping means,according to the present invention,

Figure 2 is a section on the line 2-2 of Figure 1,

Figure 3 is a diagrammatic plan view of the general assembly, and IFigures 4 to 6 are each sectional side elevations of modified forms ofthe gripping means, shown in Figure 1.

In carrying the-invention into effect according to one convenient modeby way of example, as applied to the construction of a prestressedconcrete member by the post-tensioning method,.a 'prestress bar L havingribshaped surface deformations 2,- is located in a duct 3 formed in theconcrete member 4, see Figure l. The bar 1 is anchored in a grippingdevice, the subject of the present invention, indicated generally at 5,see Figure 3, to the opposite end of which a jack 6 is connected by itsshaft 7 as described hereafter. The jack 6 is braced against the endface '8 of the concrete member 4 by legs 9 and has the usual jackoperating means, hydraulic or mechanical (not shown). The arrangementshown in Figure 3 is diagrammatic only to show the general arrangementand application of the gripping mean-s according to the presentinvention.

The bar 1 is gripped in the gripping device 5' by wedges 10, which asshown may be provided with grooves 11 to accommodate the deformations 2and provide inter-'- lock while at the same time enabling the wedges tohavemajor surface contact with the surfaceproper 1a of the bar, or thewedges may have serrated faces (not shown) which cut into thedeformations and surface of the The wedges 10 may be two in number, asshown in Figure 2, and be contained in a tapered hole 12 of rectangularcross section in a wedge collar or block 13 (hereinafter referred to asthe wedge collar 13), or may be more than two in number and contained ina hole of conical form (not shown) in which case the wedges maypreferably be substa'ntially' a' lit to the circular form of the hole.The Wedge collar 13 is externally of cylindrical form and is threadedconcentrically with its axis (as described in' detail hereafter). Thethreading of smaller diameter, indicated at 14 is formed on a portion ofthe wedge collar 13 of reduced diameter compared with that of the otherthreaded portion, indicated at 15 and is located at the end of the wedgecollar 13 which will be disposed remote from the face of the concretemember. This threading 14 (hereinafter referred to as the innerthreading 14) is for the purpose of connecting the gripping device tothe jack, directly or indirectly to the jack shaft 7. Thus an adaptor16, in the form of a pipe or sleeve, is threaded at one end over theinner threading 14 and at its other end has threaded connection with thejack shaft 7, if necessary through an externally and internally threadedcollar 17.

The threading 15 on the larger diameter (hereinafter referred to as theouter threading 15) is for the purpose of engaging an internallythreaded thrust collar or sleeve 18 which can be adjusted axiallylengthwise of the wedge collar 13, and of the bar 1 as the bar istensioned so as to maintain its bearing on the face 8 of the concretemember (or hearing plate 19), where such is provided, as shown inFigures 1, 2 and 3. The bearing plate 19 may have a rubber or otherresilient backing 20 interposed between it and the face 8 of theconcrete member and may be large enough also to provide a bearing forthe thrust of the jack through the jack legs 9, as shown particularly inFigures 2 and 3. The bearing plate 19 may further be thick enough tohave a grouting hole 21 formed therein to allow for the force of groutin the duct space 3 around the bar 1 in the concrete member 4. Toprevent the escape of grout outwardly, the bearing plate 19 may have arubber or other washer 22 located in a recess 23 aroundthe axis of thebar 1, which is held in position and tightened against the bar by agland ring 24 threaded into the recess 23.

In use the procedure is to place the bearing plate 19 and backing 20over the end of the bar, fit the washer 22 and tighten the gland ring24. The wedges 10 are then placed on the bar and secured thereon withthe wedge collar 13, close to the bearing plate and with the thrustcollar 18 screwed back. The jack 6 is then placed in position andconnected to the wedge collar 13 by the adaptor 16 (or in thealternative ways hereinafter described) with its legs 9 bearing on thebearing plate 19.

When the jack 6 has caused the bar 1 to be fully tensioned and extended(or during this process) the thrust collar 19 is screwed up andtightened against the bearing plate 19, so that when tension is relievedfrom the jack and the latter removed the bar 1 is held in tension andextension until the bar duct 3 is grouted (for example through thethrust plate grouting hole 21) and the grout has hardened.

The thrust collar 18 is then screwed back, the jack 6 being re-appliedif necessary, to make unscrewing easier, and the gripping device 5removed entirely by dismantling, after which the projecting end of thebar 1 may be cut off.

For use in the production of prestressed concrete members by thepretensioning method, the procedure dc scribed above can be used wherethe moulds themselves are sufficiently strong to resist the reaction totension of the prestress bar or bars. Further no provisions for groutingare required since the concrete bonds directly with the bar or bars andusually the bearing plate 19 can be omitted as the end of the mould isnormally such as to be able to take the thrust from the jack itself andthrust collar 18 unaided.

When manufacturing pretensioned concrete by the socalled long linemethod, the gripping devices 5 according to the present invention, maybe placed at one or both ends of each prestress bar and extensions ordraw bars may be used to connect the gripping devices at one end withthe fixed abutment of the pretensioning bed and at the other end withthe thrust plate to which the jack or jacks apply the tension to thebars in known manner. Theextensions or draw bars are therefore attachedto 4 the gripping devices, according to the present invention, in thesame or similar ways as the jack may be attached, as described above andhereafter, but the thrust collar 18 is not needed and therefore theouter threading 15 on the wedge collar 13 is not employed.

The application of the gripping device, according to the presentinvention, to the production of prestressed concrete by the pretensionedmethod has been described herein to show pretensioning andpost-tensioning methods and is therefore doubly useful.

According to a modified form of a gripping device described withreference to Figures 1 and 2, the inner threading by which the jack isconnected to the gripping device may be formed on an inner recessedsurface as opposed to an externally facing surface of reduced diameter.Thus, see Figure 4, the wedge collar 13a is provided with a co-axialrecess or bore 30, which is internally threaded at 14a to engage a jackadaptor sleeve or pipe i6a connected to the jack shaft 7 as previouslydescribed. In order to support the portion 31 of the wedge collar 13aagainst outward radial pressure auxiliary threads are provided on theouter surface of the wedge collar to engage the thrust collar 18 incontinuation of the outer threading 15, a portion 32 being relieved ofthread ing to reduce the interthread friction.

According to a still further modification shown in Figure 5, innerthreads 14a may again be provided on a recess or bore 30 to engage witha jack adaptor pipe or sleeve 1612, the wedge collar 1311 being providedwith a long continuous outer threading 15b on its external peripherywhich engages a thrust collar 18a of very short axial extent comparedwith the thrust collar 18 previously described. in order to bridge thegap between the short thrust collar 18a and the face 8 of the concretemember (or the thrust plate 19) a plain intervening member 18b isprovided as a distance piece therebetween.

The previously described method of coupling the inner threading 14 or14a to the jack adaptor sleeve or pipe may comprise providing a jackadaptor sleeve or pipe 16b having therein a slot 33 through which acotter 34 can pass to engage a slot 35 in the jack shaft 7a.

1 claim:

1. A device for stressing reinforcing bars in concrete that comprises acollar member adapted to surround an end of a reinforcing bar, a taperedinner surface on said collar, tapered bar-encircling wedges adapted tofit between said collar and the reinforcing bar to grasp said bar, athreaded outer surface on said collar, an internally threadedtension-retaining sleeve received on the threaded portion of said collarand serving to retain the tension in said reinforcing bar after it isapplied by the. device, a second threaded surface on said collar oflesser diameter than the first, and a threaded tension member receivedupon said second threaded portion and serving to adjustably connect saidcollar to a tension-producing means.

2. A device for stressing reinforcing bars in concrete as claimed inclaim 1, wherein a pair of wedges are provided which are substantiallyflat on all but their barcontacting surfaces and are accommodated in thewedge collar in a tapering aperture of corresponding cross section.

3. A device for stressing reinforcing bars in concrete as claimed inclaim 1, wherein the plurality of wedges have arcuate outer surfaces incross-section so as to gether to make up a circular cross section andare accommodated in the wedge collar in a tapering apertureof circularcross-section.

4. A device for stressing reinforcing bars in concrete as claimed inclaim I, wherein the bar-engaging surfaces of the wedges are grooved toconform with rib-shaped deformations provided on the prestress bar so asto provide interlock and major surface contact between the wedges andthe bar surface proper.

5. A device for stressing reinforcing bars in concrete,

5 as defined in claim 1, wherein the tension member is connected to thetensioning device shaft by a slot and cotter arrangement.

6. A device for stressing reinforcing bars in concrete in accordancewith claim 1, wherein the sleeve threaded on to said outer threadedsurface of the wedge collar is of sufiicient axial length to extend froma wedge collar to said thrust supporting surface associated with theconerete. Y 7; A device for stressing reinforcing bars in concrete, asdefined in claim 1, wherein the sleeve threaded on to said outerthreaded surface of the wedge collar is of short axial length and anon-threaded extension is provided between the sleeve and a thrustsupporting surface associated with the concrete.

8; A device for stressing reinforcing bars in concrete, as defined inclaim 1, in which the second threaded surface on the wedge collar is anexternal surface of reduced diameter.

9. A device for stressing reinforcing bars in concrete, as defined inclaim 1, in which the second threaded surface on the wedge collar is aninternal surface of lesser diameter than the threaded outer surface.

10. A device for stressing reinforcing bars in concrete, as defined inclaim 1, in which the threaded tension member is disposed coaxially withthe bar to be tensioned and is connected to a jacking device forapplying tension to the reinforcing rod.

References Cited in the file of this patent UNITED STATES PATENTS1,597,071 Eubanks Aug. 24, 1926 1,606,789 Hooley Nov. 16, 1926 FOREIGNPATENTS 951,592 France Apr. 18, 1949

